Ionic hydrocarbon copolymers are added to polyalkylene terephthalate resins for two reasons. First, the ductility of articles molded from the resin can be enhanced by adding an ionic hydrocarbon copolymer of an .alpha.-olefin and an .alpha.,.beta.-ethylenically unsaturated organic acid. Second, molded articles having smooth glossy surfaces can be obtained at unusually low mold temperatures for polyethylene terephthalate (i.e., below 110.degree. C.) if certain materials (such as low molecular weight esters, ketones, amides, etc.) are added to the polyethylene terephthalate in conjunction with a selected ionic hydrocarbon copolymer.
However, presence of the ionic hydrocarbon copolymer causes articles molded from the resin to discolor somewhat as the article is subjected to heat in the presence of air. An agent which inhibits such darkening is desirable.